In late years, a lithium secondary battery capable of providing a high electromotive force through an oxidation-reduction or redox reaction in lithium has become widely accepted as a new type of high-power, high-energy density battery. This type of lithium secondary battery typically comprises a cathode made of metal oxide, such as cobalt, nickel, manganese, iron, vanadium or niobium oxide.
The cathode made of such a metal oxide, however, involves problems of increase in weight and cost, and a small number of reactive electrons which often leads to an insufficient capacity per weight, resulting in difficulty in obtaining a high-capacity, high-energy density lithium secondary buttery.
In connection with recent studies of using a conductive polymer as electrochemical elements and utilizing such electrochemical elements as an electrode for lightweight, high-energy density batteries, a large-area electrochromic element, or a microelectrode for biochemical sensors, the research has been made on the use of a conductive polymer, such as polyaniline, polypyrrole, polyacene or polythiophene, as a battery electrode.
For example, U.S. Pat. No. 4,833,048 discloses an organosulfur compound for use as a cathode material intended to provide a high-capacity, high-energy density polymer battery. This organosulfur compound serves as an electrode material capable of a reversible reaction such that the S—S bond of an organodisulfide compound is broken through electrolytic reduction to form organic thiolate, and the organic thiolate is electrolytically oxidized to re-create organic disulfide.
The actual research has been made on obtaining a high-energy density lithium secondary buttery by using organosulfur compounds as a cathode material allowing discharge and recharge through a redox reaction in sulfur. However, in use under room temperature, the organosulfur compound exhibits a low redox reaction rate, and a sufficient current cannot be picked up from itself. That is, the organosulfur compound is inherently an insulating material having a low reaction rate at room temperature, and is thereby limited to use at a high temperature of 100° C. or more. In addition, during reduction reaction (discharge period), the organosulfur compound is in a low molecular weight state, and dissolved/diffused out of the electrode, which leads to deterioration in the efficiency of electrode reaction.
In order to solve the above problems, a technique of using a conductive polymer in combination with an organosulfur compound is proposed, as disclosed, for example, in Japanese Patent Laid-Open Publication Nos. H04-264363, H04-272659, H04-359866, H05-6708, H05-82133, H05-135767, H05-135768 and H05-135769, and U.S. Pat. No. 5,324,599.
Japanese Patent Laid-Open Publication No. H06-231752 discloses an electrode prepared by combining 4,5-diamino-2,6-dimercaptopyrimidine, particularly among disulfide compounds, and a conductive polymer with a π-electron conjugated system, and Japanese Patent Laid-Open Publication No. H07-57723 discloses an electrode prepared by combining 7-methyl-2,6,8-trimercaptopurine, particularly among disulfide compounds, and a conductive polymer with a π-electron conjugated system.
Japanese Patent Laid-Open Publication Nos. H05-74459, H05-314979 and H06-283175 disclose an electrode material comprising a conductive polymer with a disulfide group, an electrode material comprising an organosulfur aromatic compound prepared by introducing a sulfur atom into an aromatic carbon atom, and an electrode material comprising an organodisulfide compound polymer consisting of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) or homopolymer of thiocyanuric acid or copolymer thereof, respectively.
In particular, an electrode using a complex of an organosulfur compound and polyaniline or a conductive polymer playing a role of increasing the redox rate of organodisulfide is disclosed in Japanese Patent Laid-Open Publication Nos. H08-213021, H08-222207, H09-82329, H09-106820 and H10-27615. It is also known that a complex of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) and polyaniline can be used as an organosulfur compound-based cathode material for secondary batteries capable of adequately operating even at room temperature (“Contemporary Chemistry” October 1996, pp 34–41).
However, this complex cannot completely suppress the deterioration in capacity because it is not a newly created compound capable of inducing a certain chemical bond. In addition, the separation between polyaniline and DMcT would occur in the electrode to cause deterioration in the mobility of electrons and electrode reaction rate.
There have also been known some techniques for improving the cycle characteristics of an organodisulfide electrode, such as the use of metal complex of organodisulfide (U.S. Pat. Nos. 5,516,598 and 5,665,492, and Japanese Patent Laid-Open Publication Nos. H09-259864, H09-259865, H10-241661 and H10-241662), and the use of an positive material comprising a mixture of a conductive polymer and a lithium thiolate compound with S—Li ionic bond which will form S—S bond through electrolytic oxidation (Japanese Patent Laid-Open Publication No. 05-314964).